The exemplary embodiments of the invention generally relate to ground fault sensing devices and more particularly to setting a ground fault trip response function (shape of the response) in a trip unit or other overcurrent protective device with automatic or controlled operation.
Circuit breakers are used for protecting electrical conductors such as cables and bus bars in equipment. The circuit breaker trip mechanism monitors current through an electrical conductor and “trips” the circuit breaker to open the electrical circuit to interrupt current flow through the circuit provided that certain predetermined criteria are met. The circuit breaker may, of course, also be used to monitor voltage, and trip in case of any disturbance in pre-set voltage conditions such as under-voltage, over-voltage, and voltage imbalance conditions. Other trip criteria can include, for example, the maximum continuous current permitted in the protected circuit.
Overcurrent protective functions are usually designed as inverse time functions with a “pickup” threshold and a response curve that relates how much over the threshold current the monitored current is, and how long it lasts over the threshold current. A monitored current that is only slightly above a threshold current may be allowed to continue longer than a current that greatly exceeds the threshold current value. As long as the monitored current remains below any protection rating (long-time, short-time, ground fault, or instantaneous), the circuit breaker will remain closed.
An electronic trip unit (“ETU”) is a device that is used in conjunction with an electro-mechanical circuit breaker to control the current (or voltage) versus time trip response. The time current curve characteristics define the trip time and currents permitted by the circuit breaker. Circuit breaker characteristics can be set by the user via a variety of threshold (pickup) settings and response shaping parameters to provide a specific inverse time protective function, also referred to herein as the “time current curve”. As long as the monitored current remains below the various threshold settings and the associated time current curve characteristics, the circuit breaker or overcurrent device will remain closed. Momentary low magnitude-excursions above the threshold (pickup) settings are tolerated as governed by the time current curve characteristics or response function associated with the pickup settings. However, persistent overcurrents, or currents in excess of threshold settings that last long enough to engage the time current curve will result in the tripping of the circuit breaker.
Circuit breakers are designed to trip on this basis so as not to trip in response to normal, momentary currents which flow, for example, during motor starting or transformer energization. Overload current responses of fuses and overload relays are also predicated on an inverse-time basis. Circuit protective devices, including circuit breakers, are also designed to respond to overcurrents of short circuit proportions, e.g., ten times rated current, on an instantaneous basis, that is, without intentional delay. A typical circuit breaker clearing time, i.e., the maximum time taken by the breaker to physically open its contacts and interrupt short circuit currents, is typically in the range of eight to fifty milliseconds.
Intermediate the overload and short circuit overcurrent ranges is a heavy overload current range, e.g. three to ten times rated current, which is typically handled on a fixed time delay basis. That is, circuit interruption in response to heavy overload current levels is effected upon the expiration of a predetermined fixed short time delay. This portion of the protection may be designed as a “definite time” function, which means that once the threshold is exceeded the response is strictly dependant on time, not current magnitude. Alternatively, the response may have an inverse time response characteristic. The most common example of this inverse time response characteristic is referred to as an “I2t slope” defined by the function {Time to trip=k/I2}, where k is a constant that defines the location of the slope on the time-current curve.
Once an electrical power distribution system has been designed, its loads and operating conditions defined, and the voltages and electrical equipment selected, it remains to determine the appropriate circuit protective devices to be used. The primary concern in this determination is to prevent or at least minimize damage to the conductors within the distribution system, and possibly connected loads, in the event of a fault or overload precipitating abnormal overcurrent condition. Such a fault may be caused by equipment failure, human error, or emergencies of natural origin. Typically, such faults are unpredictable, and thus the selected circuit protective devices must function automatically to isolate the fault from the rest of the system, minimize damage to conductors and adjacent equipment or conductors and, incidentally, to minimize hazard to other property and personnel that may be in physical proximity to the fault location.
Another principal concern associated with the determination of the circuit protective devices to be used is to minimize the extent and duration of electrical service interruption in the event of a fault. In all but the simplest systems, there are two or more circuit protective devices between a fault and the source of the fault current. In order to minimize electrical service interruption, overcurrent protective devices feeding the faulted circuit should respond in a particular order. The device feeding the faulted circuit closest to the fault should trip to minimize unnecessary power interruption to other portions of the power distribution system. If, for any reason, this protective device does not clear the fault in timely fashion, the next upstream protective device will attempt to do so in a back-up role, and so on. A series of overcurrent protective devices selected and set to provide this “selective” mode of operation is said to be “selectively coordinated”. To achieve such coordination the protective devices must be selected and set on the basis of their particular time current curve response characteristics so as to operate on the minimum current that will enable them to fully carry both steady state and transitory rated circuit current while responding to undesirable levels of current as quickly as possible. Each device should operate in the minimum time possible and yet be selective, i.e., coordinate, with other devices in series therewith. When these two requirements are met, damage to the system and service interruption are minimized. However, in many cases some level of compromise between optimal selectivity and optimal protection must be accepted. Engineers practiced in the art of selecting and setting circuit breakers and associated trips are often called to determine how much selectivity to compromise to achieve better protection, or visa versa.
Conventional electric circuits normally carry balanced electrical currents, with the return current from an electrical load flowing through a neutral conductor. Unless a ground fault current is flowing, the phase and neutral currents of a branch circuit sum to zero. In the event of a ground fault, the phase and neutral currents do not sum to zero, and the difference between the phase and neutral currents is the ground fault current.
One type of ground fault that may not be sufficiently well protected by normal trip unit overcurrent functions is a fault type known as an “arcing ground fault”. Arcing ground faults may have some specific characteristics that make them particularly damaging and difficult to detect via normal sensing designed for phase overcurrent protection. An arcing ground fault current is one that involves current flowing through air causing ionization of the same air allowing current to flow. This “arc” may create sufficient impedance in the form of resistance to keep the fault current relatively low. This low, but still damaging, current may be difficult to detect unless the overcurrent protective device trip systems employs a specific ground fault protective function.
The ground fault protective function is conventionally executed by measuring the individual phase currents and the neutral current of a circuit and summing the measured currents to identify any difference between the phase and neutral currents. A single current transformer, such as a “zero-sequence” transformer is conventionally used to measure the phase and neutral currents and identify any differential or ground fault current. Once the ground current is measured it may be compared to its own dedicated pickup threshold settings and ground-fault inverse time overcurrent function which is different from the similar function assigned to the phase conductor protection. Typically the ground-fault protection is set to be more sensitive than phase protection. Several industry standards such as NFPA 70 (a.k.a. National Electrical Code [NEC]) and UL 1053 strictly regulate when dedicated ground fault protection must be implemented within a power distribution system and various parameters that define required sensitivity and operating time constraints for ground-fault protection.
The constraints described within the various standards create significant complexity and provide for significant limitations for the “selectivity” described as desirable in the previous sections of this document. Traditional ground fault overcurrent response shapes do not provide sufficient flexibility to optimize protection and selectivity while still adhering to the performance limitations created by applicable standards. It would be advantageous to provide greater curve shaping flexibility than is currently available in the industry to meet all of these conflicting requirements, as well as the applicable standards.